Organic electroluminescent (EL) devices contain spaced electrodes separated by an organic light-emitting structure which emits light in response to the application of an electrical potential difference across the electrodes.
The basic form of the organic EL devices is a two-layer structure, in which one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for hole-electron recombination and resultant electroluminescence. Examples are provided by U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; 4,885,211; 4,950,950; 5,047,687; 5,059,861; 5,061,569; 5,073,446; 5,141,671; 5,150,006 and 5,151,629.
The simple structure can be modified to a three-layer structure, in which an additional luminescent layer is introduced between the hole and electron transporting layers to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. For instance, the luminescent or recombination layer can be chosen to have a desirable EL color as well as a high luminance efficiency. Likewise, the electron and hole transport layers can be optimized primarily for the carrier transport property.
It is well known that operating voltage of the organic EL devices can be significantly reduced by using a low-work function cathode and a high-work function anode. The preferred cathodes are those constructed of a combination of a metal having a work function less than 4.0 eV and one having a work function greater than 4.0 eV, as disclosed by Tang et al in U.S. Pat. No. 4,885,211 and by VanSlyke et al in U.S. Pat. No. 5,059,062. The use of a LiF/Al bilayer to enhance electron injection in organic EL devices has also been disclosed by Hung et al in U.S. Pat. No. 5,677,572.
In organic EL devices, the anode is commonly formed of indium tin oxide (ITO) because of its transparency, good conductivity, and high work function. However, a light-emitting structure grown on a bare ITO surface generally shows poor current-voltage characteristics and low operational stability. The mitigation of these problems has involved introducing an intermediate layer between the ITO and the organic light-emitting structure. For instance, VanSlyke et al. demonstrated a highly stable organic device formed by using a multilayer structure with a CuPc-stabilized hole-injection contact. See "Organic electroluminescent devices with improved stability" by S. A. VanSlyke, C. H. Chen, and C. W. Tang, Applied Physics Letters, Vol. 69, 2160 (1996). However, the CuPc layer interposed between the ITO and a hole-transporting layer results in a substantial increase of the drive voltage because of a hole injection barrier present at the interface between the CuPc and the hole-transporting layer NPB.
Thin organic layers are generally formed by using spin coating or thermal evaporation. It was found recently that the layers can also be fabricated from organic vapors subjected to glow discharge. Cross-linked polymers are generated in plasma, and the process is usually called "plasma polymerization". The unique feature of plasma polymerization is the formation of a ultra-thin layer with a minimal amount of flaws. With respect to spin coating or vacuum deposition plasma polymerization provides layers with excellent conformality, sufficient durability, and improved adhesion.
Thin organic layers prepared by plasma polymerization at radio frequency (RF) are, in general, dielectric materials with insulating properties and extremely low conductivities. For tetrafluoroethylene polymerized by plasma, Vollman and Poll (see Plasma Polymerization by H. Yasuda) reported dc conductivity values in the range 10.sup.-17 -10.sup.-18 (ohm-cm).sup.-1. Hetzler and Kay (see Plasma Polymerization by H. Yasuda) reported ac conductivities between 10.sup.-15 and 10.sup.-10 (ohm-cm).sup.-1 measured at 10.sup.-3 -10.sup.5 Hz. For very low frequencies, however, the conductivity leveled off to the dc value at 10.sup.-17 (ohm-cm).sup.-1, which is in good agreement with Vollman and Poll's data. Because of the low conductivities, the materials are commonly used as dielectrics or corrosion protective coating.